Field of the Invention and Related Art
The present invention relates to an electrophotographic image forming apparatus such as a copying machine or printer.
FIG. 6 illustrates an example of the general structure of an electro-photographic color image forming apparatus.
This color image forming apparatus comprises: a photosensitive drum 1 as an image bearing member, which is rotatively supported and is rotatable in the direction indicated by the arrow; a photosensitive drum 1 which is surrounded with a charger which uniformly charges the surface of the photosensitive drum 1 (in this case, a corona type charger); an image exposing means 3 that forms an electrostatic latent image correspondent to image data on the photosensitive drum 1; a developing apparatus 4 which develops the electrostatic latent image formed on the photosensitive drum 1 into a visible image; a transferring apparatus comprising a transfer drum 5a as a recording medium carrying member; a cleaner 6 which removes the developer remaining on the photosensitive drum 1, and like members.
The image exposing means 3 in this example is a laser beam based exposing apparatus comprising a polygon mirror, a lens system, and the like. The reflected light coming out of an unillustrated section for scanning an original is separated with a color separation filter or the like into primary color images. The thus obtained primary color images are converted to color image signals. A laser beam, which is modulated by each of the color image signals in a laser beam emitting section and is emitted therefrom, is projected onto the peripheral surface of the photosensitive drum 1 (for example, 82 mm in diameter) to expose the surface with an optical image E, or its equivalent, forming thereby an electrostatic latent image corresponding to each of the color image signals.
Also, in this example, a rotary developing apparatus is employed as the developing apparatus, in which four developing devices, for example, a developing device 4K for black color, a developing device 4C for cyan color, a developing device 4M for magenta color, and a developing device 4Y for yellow color, are mounted on a rotary member 4a, which rotates about a central axis 4b. Placement is such that a desired developing device is rotatively moved to a developing station, where it directly faces the photosensitive drum 1 to develop the latent image on the photosensitive drum 1.
Here, an overall operational sequence of this color image forming apparatus will be briefly described with reference to a full-color mode. First, the photosensitive drum 1 is uniformly charged by the charger 2. Then, the photosensitive drum 1 is exposed to a laser beam modulated by the image signal corresponding to the cyan color component of the original, whereby an electrostatic latent image is formed on the photosensitive drum 1. This latent image is developed by a cyan developing device 4C, which is moved in advance into the developing station. As a result, a visible cyan image (toner image) is formed on the photosensitive drum 1 by the resin based toner.
Meanwhile, recording medium such as transfer sheet is fed with a predetermined timing from a recording medium cassette 7a, 7b or 7c, or a manual feeding section. The sheet is then conveyed through a sheet delivery path indicated by a dotted line in the drawing, by a sheet delivery system comprising a pickup roller, a sheet feeding guide, a sheet feeding roller, and the like, to the transferring apparatus 5, in which it is wrapped around a transfer drum 5a.
This transferring apparatus 5 in this example comprises: a transfer drum 5a (for example, 164 mm in diameter) as a recording medium carrying member; a corona type charger 5b which discharges the corona for transferring the toner image from the photosensitive drum 1 onto the recording medium; a corona type charger 5c as means for attracting and adhering electrostatically the recording medium to the transfer drum 5a; a roller (contact type) 5g as an opposing electrode of the sheet attracting corona type charger 5c; an internal charger 5d of the corona type; and an external charger 5e of the corona type. The transfer drum 5a is supported so as to be rotatively driven and its cylindrical periphery is covered with a sheet 5f of dielectric film, which serves as means for carrying the recording medium.
The transfer drum 5a is rotated in the direction of the arrow in the drawing in synchronism with the photosensitive drum 1, and the latent image corresponding to the cyan color is developed into a cyan toner image by the cyan image developing device 4C. As the transfer drum 5a is further rotated, the visible cyan toner image is moved to the transfer section, where it is transferred onto the recording medium carried on the recording medium carrying sheet 5f by the corona type transfer charger 5b.
Thereafter, the transfer drum 5b continues its rotation, being on standby for the transfer of the next color image (for example, a magenta toner image).
As for the photosensitive drum 1 from which the toner image has been transferred, it is cleared of deposits such as the residual toner by a cleaner 6. Photosensitive drum 1 is then uniformly charged again by the charger 2, and is subjected to an imaging exposure by the laser beam modulated by the image signal corresponding to the next color, magenta, in the same manner as described before. Meanwhile, the developing apparatus 4 is rotated such that the magenta color developing device 4M is positioned at a designated developing point and develops the latent magenta image into a magenta toner image.
Then, this magenta toner image is moved into the transfer section, where it is transferred by the transfer charger 5b onto the recording medium carried on the recording medium carrying sheet 5f, in such a manner as to be overlaid on the cyan toner image. The transfer drum 5a continues its rotation, being on standby for the transfer of the next color toner image (i.e., yellow).
Next, the image forming and transferring processes described above are carried out for the yellow and black colors to finish overlaying the four color toner images. Thereafter, the recording medium is discharged by separating charger 5h, and then, is separated from the transfer drum 5a by a separating means 8 such as a separating claw. The separated recording medium is conveyed through a sheet path indicated by a dotted line in the drawing, and delivered to a fixing device (in this example, a heat roller type fixing device), by appropriate conveying means. In the fixing device, the four color toner images are fixed all at once, and then, the recording medium with the fixed full-color image is discharged onto the external tray.
The process described above is a single cycle of full-color image formation for producing a sheet of desired full color print.
However, a conventional image forming apparatus employing the above described structure suffers from the following faults.
(a) It takes substantial time to position a pertinent developing device into the developing station, which translates into a longer time for completing the image formation. This is because the developing apparatus is a rotary developing apparatus, which has to be rotated to position properly the developing device.
For example, when a sheet of A3 size paper, or two sheets of A4 size or letter size (LTR), which are the typical recording medium, are adhered on the recording medium carrying sheet 5f of the transfer drum 5a in a well-known manner, the interval between the sheets is short; therefore, it often happens that by the time the pertinent developing device of the developing apparatus arrives at the developing station, the latent image to be developed, which is on the photosensitive drum 1, will have passed the developing station, that is, the pertinent developing device is late for the intended image development. As a result, the latent image cannot be developed until it arrives at the developing station the next time. In other words, the photosensitive drum 1 must undergo an extra rotation, which prolongs the time between the beginning of an image forming operation and the end thereof.
(b) When a developing apparatus comprising a developing device used with two component developer, which will be described later, is employed, the toner and carrier are liable to be separated and scattered. More specifically, in the rotary developing apparatus, the toner is liable to be scattered from the developing device having been moved to a downward position, relative to the gravity direction. This results in contamination of the interior of the image forming apparatus.
(c) The photosensitive drum is liable to be damaged by being repeatedly exposed to the transfer charge since the diameter of the photosensitive drum is small. More specifically, because of commercial and industrial reasons such as the cheaper cost and less harmful properties, a photosensitive drum containing organic photosensitive member (hereinafter, OPC drum) is used, which is normally charged with a negative polarity to form a latent image, as opposed to being subjected to a positive charge during the image transfer. When the drum is subjected to the transfer charge of the positive polarity without the interposition of the recording medium, its charge polarity is liable to be shifted to the positive side.
Generally speaking, once the OPC drum being used on the negative polarity side is charged to the positive polarity, it is difficult to reverse the charge characteristics back to the negative side, and even if is reversed, the charge obtained thereafter becomes unstable, which is liable to fail to produce stably preferable images.
Because of the reasons described above, an image forming apparatus comprising a photosensitive drum with a larger diameter and fixed developing devices has been proposed.
Referring to FIG. 2, an example a of conventional electro-photographic color image forming apparatus having such a structure as described in the foregoing will be described. In FIG. 2, the members, components, elements or the like, which are equivalent to those in the color image forming apparatus illustrated in FIG. 6, will be given the same reference numerals as the ones given in FIG. 6. FIG. 2 illustrates only the photosensitive drum 1 and transfer drum 5a, omitting the remaining structural which are the same as those in FIG. 6.
In this exemplary color image forming apparatus, the image bearing member is a photosensitive drum 1 with a diameter of 180 mm, being supported so as to be rotatively driven in the direction of the arrow in the drawing. There are also provided in this apparatus: a developing apparatus consisting of four developing devices 4Y, 4C, 4M and 4K, each of which contains a developer of different color for developing an electrostatic latent image formed on photosensitive drum 1 into a visible image; a transferring apparatus comprising a transfer drum 5a as the recording medium carrying member; and the like.
Next, the developing operations of the developing devices 4Y, 4C, 4M and 4K within the aforementioned structure will be described in detail. In this structure, the developing devices 4Y, 4C, 4M and 4K are arranged in this order around the photosensitive drum 1 as the image bearing member from the upstream side towards the downstream side relative to the moving direction of the photosensitive drum 1 (rotational direction). When an operation for developing sequentially each of the latent images formed on the photosensitive drum 1 is carried out, the developing devices are sequentially activated from the downstream side toward the upstream side.
In other words, the order in which the developing devices are activated is different from that in which they are arranged; the devices are activated in the order of the magenta, cyan, yellow and black colors. That is, the developing operation is initiated from the magenta developing device 4M and is sequentially moved upstream relative to the rotational direction. Then, the operation is moved from the yellow developing device 4Y to the black developing device 4K, or, from the most upstream side to the most downstream side. Therefore, it cannot be called "from the downstream side to upstream side," in a strict sense. However, this movement is nothing but a movement for switching from one device to the next device; therefore, practically speaking, it may be called "in order from the downstream side to -upstream side."
Further, during a continuous image forming operation, this operational order of the developing devices is maintained so that the developing devices are always activated from the downstream side toward upstream side relative to the rotational direction of the photosensitive drum.
With the aforementioned structural arrangement, the actual yellow toner portion of the two component yellow developer from the yellow developing device 4Y, which is the one on the most upstream side, is scattered and mixed in the cyan, magenta and black colors from developing devices 4C, 4M and 4K, which are on the downstream side. However, since the yellow color is weak in terms of visual sensitivity, none of the problems that might have been caused by color mixing is found in the finished image.
Further, the color mixing caused by the scattering of magenta or cyan toner from the magenta developing device 4M or the cyan developing device 4C, respectively, does not create any problem in the finished image since it also is weak in terms of the visual sensitivity.
Thus, with the employment of the structure described above, it is possible to form a preferable image without suffering from any problem that might arise due to color mixing during the image forming operation.
Further, even during a full-color developing cycle, in which a sheet of A3 size recording medium or two sheets of A4 size recording medium are placed on the drum and the developing devices are activated from the downstream side toward the upstream side, there is no need for the transfer drum idling, which is a shortcoming of the conventional apparatus illustrated in FIG. 6.
The points described in the foregoing will be described in further detail with reference to the apparatus illustrated in FIG. 2 and the A3 size sheet. As described above, when a sheet of A3 size recording medium P is delivered to the transfer drum 5a, an a mm wide (in the rotational direction of the drum) strip of the peripheral surface of the transfer drum 5a is not covered with the recording medium. This distance a is longer than any of the distances between the adjacent two developing devices: b1 mm (between the black developing device 4K and magenta developing device 4M); b2 mm (between the magenta developing device 4M and cyan developing device 4C); b3 mm (between the cyan developing device 4C and yellow developing device 4Y).
First, when each of the color toner images is transferred onto a sheet of A3 size recording medium, the developing operation starts from the magenta developing device 4M and moves upstream to the cyan developing device 4C and to yellow developing device 4Y. Then, it is moved from the yellow developing device 4Y, which is the most upstream device, to the black developing device 4K, which is the most downstream one, where a cycle of the full-color image development ends. During the sequence of forming these toner images, the developing devices have been switched while the transfer drum 5a correspondingly rotates (a-b2) mm, (a-b3) mm, or (a+b1+b2+b3) mm; therefore, there will be no problem.
Next, when the second sheet of the recording medium is consecutively fed during the continuous copying operation, there is an interval of only a mm between the trailing edge of the first sheet and the leading edge of the second sheet, as described before. However, in this apparatus, the developing operation starts from the magenta developing device 4M on the most downstream side and is sequentially moved upstream to the cyan developing device 4C, to yellow developing device 4Y, and finally, is switched back from the yellow developing device 4Y on the most upstream side to the black developing device 4K on the most downstream side, repeating thereafter the same sequence. Therefore, the moment when the developing operation is switched from the black developing device 4K to magenta developing device 4M, the trailing end of the black latent image on the photosensitive drum 1, that is, the trailing end of the last latent image for the first sheet, comes out of the black developing device 4K, the leading end of the first latent image, that is, the magenta latent image, for the second sheet is at a point (a-b1) mm upstream of the magenta developing device 4M. This distance of (a-b1) mm gives sufficient time for making the switch between the developing devices, eliminating the need for one extra rotation of the transfer drum 5a.
In other words, it is unnecessary to waste time for the transfer drum 5a idling for each color as it is in the case of the rotary developing system illustrated in FIG. 6. As a result, mechanical performance deterioration such as a reduction in copying speed does not occur.
In addition, it is unnecessary to involve two or more developing devices at the same time to carry out the developing operation. Therefore, only a single motor M is needed in order to carrying out the developing operation, wherein the motor M is required to drive only one developing device at a time through one of the clutches CY, CC, CM or CK for each developing device. In other words, it does not happen that while one developing device is developing the pertinent latent image, another one is driven, causing thereby a torque change. Therefore, the latent images can be preferably developed.
However, even the image forming apparatus employing the fixed developing system such as the one illustrated in FIG. 2 suffers from the following shortcomings.
In the case of the two component developer, the potential difference necessary for obtaining an appropriate density varies for each developer, and this necessary potential difference is determined by the charge characteristics and amount of the pigment content in each developer. When the charge characteristics are relied on to equalize the potential for obtaining the proper density, the developer material properties must be substantially changed, which is extremely difficult.
Further, in terms of extending developer replenishing intervals, it is preferable to reduce the amount of developer consumption within a limit in which image quality does not deteriorate. This can be accomplished by using pigment with better dispersiveness since the better the dispersiveness the smaller a potential difference required to produce the necessary density. This in turn reduces developer consumption. However, the potential difference which each pigment requires to obtain the necessary density varies, depending on the color. However, when the pigment content of a developer that requires a smaller potential difference is reduced to increase the necessary potential difference for this developer to match the necessary potential difference for another developer that requires a larger potential difference, the developer consumption increases, which is not desirable.
Presently, generally speaking, the potential difference which each developer requires for obtaining the necessary density varies depending on the color of the pigment. Therefore, it is extremely difficult to equalize the potential difference necessary for each pigment by relying on the properties of the developer material.
Therefore, when images are formed using the image forming apparatus comprising the fixed developing system illustrated in FIG. 2, in conjunction with developers, each of which requires a different potential difference to obtain the necessary density, the toner or carrier is liable to adhere to unwanted areas when the developing devices are moved to and away from the developing station.
This problem will be described in detail, referring to the cyan and yellow developers. The yellow developer used in this conventional developing device generally displays good dispersiveness. In other words, the magnitude of the potential difference, at which a Macbeth density of 1.5 can be obtained, is 200 V, but it requires a potential difference of 350 V to obtain the same density when the cyan developer is used.
Here, "potential difference" means the difference between the potentials of the photosensitive drum and the developing device, wherein the value of the DC voltage actually applied to the developing device also varies in response to the potential difference. Next, the relation between the potential of the image portion of the photosensitive drum 1 surface and the value of the DC voltage applied to the developing device will be described with reference to specific cases: one in which the potential difference of 200 V is necessary between the photosensitive drum 1 and a developing device in order to develop reversely a latent image formed by the laser beam exposure, and another in which a potential difference of 350 V is needed under the same conditions.
As for the potential difference in this structure, a potential difference of 150 V is applied between the developing device and the photosensitive drum to attract the toner toward the developing device, so that the fog, which is created as the toner adheres to the non-image portion of the photosensitive drum 1 surface, can be prevented.
The potential is controlled so that a precise potential difference can be maintained in response to the sensitivity of the photosensitive member.
As for the developer and photosensitive material of the photosensitive member, negatively chargeable developers and a negatively chargeable OPC are employed, respectively.
When the necessary potential difference for the yellow developer is 200 V, the potential of the image portion of the photosensitive member is -50 V, and that for the non-image portion is -400 V, wherein the value of the DC voltage applied to the developing device is -250 V.
When the necessary potential difference for the cyan developer is 350 V, the potential at the image portion on the photosensitive member is -90 V, and the potential at the non-image portion is -590 V, wherein the DC voltage applied to the developing device is -440 V.
As described above, the value of the DC voltage applied to the developing device varies depending on the necessary potential difference for the developer. When the cyan color developing operation and yellow color developing operation are carried out one after another in the apparatus illustrated in FIG. 2, the value of the latent image potential on the drum 1 is that of the non-image portion. However, it is changed from the value set for the cyan to the one for the yellow at the mid point while transfer drum 5a rotates a mm.
The point at which the yellow developing device begins its operation is in a range in which the photosensitive drum 1 rotates (a-3b) mm, but the developing device is moved into, or out of, the developing station before this point. Even after the completion of the actual developing operation, the developing operation conducted by the cyan developing device is continued up until immediately before the activation of the yellow developing device to assure the reliability of the cyan color developing operation. As soon as the yellow color development begins, the cyan developing device is quickly moved away from the drum.
The yellow developing device begins its developing operation while the photosensitive drum 1 rotates (a-3b) mm. However, before it begins its operation, it must be moved into its dedicated developing station where it can develop the photosensitive member surface, and be readied for the developing operation.
Therefore, the yellow developing device comes in contact with the drum 1 before the mid point of the distance of a mm, and the cyan developing devices remains in contact with the drum 1 even after the mid point. As a result, the cyan developing device comes in contact with the non-image portion of the latent yellow image, and the yellow developing device comes in contact with the non-image portion of the latent cyan image.
The voltage applied to the cyan developing device is -440 V, whereas the potential of the latent yellow image is -400; therefore, the cyan toner is liable to adhere to the drum. The voltage applied to the yellow developing device is -250 V, whereas the potential of the latent cyan image is -590 V; therefore, the carrier is liable to adhere to the drum.
It may seem that the problem described above can be solved by improving the timing with which the developing device is moved in or out. However, it is impossible to reliably move the developing device into or out of the developing station with proper timing for the developing operation, unless extremely precise control is executed. Besides, when the image forming speed is increased, it becomes absolutely impossible.
As may be evident from the descriptions given above, it is extremely difficult to stably output a preferable image without the extra drum rotations. In other words, so far it has not been possible to output the preferable full-color image in which all of the primary colors display their maximum densities of the same level.